It is desired to provide emergency service to users of wireless communications devices, through a “911” type service (referred to as E911 or enhanced 911 service). The familiar “911” telephone number is used nationally as an emergency landline telephone number. When making a 911 call, a geolocating process is activated, and once the calling party's location has been determined, emergency personnel can be dispatched to the site. The effectiveness of emergency services depends on the ability of the emergency personnel to locate and deliver emergency services at the caller's site without undue delay. Given the widespread growth of mobile telephone use, the U.S. government has recently promulgated regulations requiring that providers of cellular telephone service provide a geolocating capability for E911 calls.
A problem with responding to a request for emergency assistance from a wireless communications device is the inability to use the calling telephone number as an aid to position location. Also, a wireless communications device user may be in unfamiliar territory and therefore unable to provide location information directly to the emergency services personnel. This lack of information regarding the caller's location hampers and delays efforts to provide emergency assistance to the caller.
A mobile cellular telephone system comprises a plurality of cells, each having a transmitting/receiving base station for sending and receiving signals from wireless communications devices, commonly referred to as cellular telephones, operating within the cell on pre-assigned frequencies. As the cell phone user moves into the coverage area of an adjacent cell, the cell phone signal is handed off from the base station of the original cell to the base station of the adjacent cell. The geographic size and shape of each cell coverage area in a cellular telephone network is determined by such factors as the beam shape and gain of the base station transmit/receive antennas, transmitter power levels, call handoff parameters and the surrounding terrain. In open flat country, cell coverage areas are substantially circular with overlapping borders. Where buildings or uneven topography block radio-frequency transmissions, the cell coverage areas may be quite irregular in shape. The cell coverage areas are designed to be smaller in urban areas to allow for closer frequency reuse, i.e., to allow more users in a given area using the same limited number of channels available to the cellular service provider.
A cellular telephone call is handed off from one cell base station to another base station based on the received signal strength or another signal quality metric determined at the base stations and/or at the cellular telephone, with the call being routed through the base station cell receiving or providing the best signal. In many areas, a number of cell sites may be capable of receiving calls from a cellular telephone, although the call is typically carried by the base station providing the highest quality service.
According to the prior art, when it is desired to determine the position of a wireless communications device in a cellular telephone system, for example, in response to a 911 call initiated from the wireless communications device, a two step process may be employed. First, the time difference of arrival (TDOA) of a signal sent from the wireless communications device and received at different base stations is determined. Second, the TDOA data and the known location(s) of the base station receiving sites are used to determine the sender's relative position.
Note that the time when the transmission of a signal begins is unknown. However, to determine the location of the wireless communications device, it is necessary to calculate the difference in the arrival times for the same signal at the several receiving sites. These time differences correspond to differences in distance between the wireless communications device and the receiving base stations, since all signals travel at the speed of light. The distance between the wireless communications device and the receiving base station is given by d=ct, where c is the speed of light (299,792,458 m/sec in vacuo), t is the transmission time and d is the distance between the wireless communications device and the receiving base station. The speed in air is essentially the same as in a vacuum. With the TDOA approach, a signal received at n receiving sites (or base stations as applied to a cellular telephone network) yields n(n−1)/2 pairs of time difference of arrival values that can be used to determine the location of the wireless communications device.
According to the prior art, the time difference of arrival of a signal received at two receiving sites is calculated in the time domain by determining the cross correlation of the two received signals. The correlation is carried out directly according to the cross-correlation integral or by the discrete time equivalent of the cross-correlation integral. Using these equivalent methods, the cross-correlation function is generated. This function represents the time offset between receipt of the signal at the two receiving sites, with the peak value of the function identifying the minimum time offset, or the highest correlation between the two signals. Thus the time difference of arrival of the signal at the two receiving base stations equals the time offset at the function peak. This is the conventional prior art time domain approach to geolocating a wireless communications device.
It is known that the signal received at each of the base stations is nearly identical, with differences typically caused by non-identical noise characteristics of the signal paths between the wireless communications device and the receiving site, multipath effects, fading and non-identical matched filters at the receiving sites. These signal differences can impact the results obtained from the cross-correlation technique described above, making the function peak difficult to locate or shifting the peak from the true TDOA value.
As illustrated in FIG. 1, according to the prior art time-difference-of-arrival technique, a wireless communications device 2 transmits a signal received at plural receiving stations 4. In the example system of FIG. 1, the plural receiving stations 4 comprise RS1, RS2 and RS3. The receiving stations RS1, RS2 and RS3 have a common time reference, such as a signal received from a high precision clock on a geosynchronous satellite, a signal received from a central processing site (not shown), or highly stable internal clocks (which can be periodically updated or synchronized to an external time reference). When an identifiable segment of the signal from the wireless communications device 2 is received, the time of receipt is ascertained at the receiving stations RS1, RS2 and RS3 using conventional techniques employing the aforementioned time reference. Information regarding the received signal, including the time of arrival of the identifiable segment, is sent from the receiving stations RS1, RS2 and RS3 to a central processing site located at one of the receiving stations RS1, RS2 and RS3 or at separate site (not shown).
At the central processing site, the signal time-of-arrival information at the receiving stations RS1, RS2, and RS3 is used to calculate the time difference of arrival of the signals. According to the prior art, the TDOA is determined by the cross-correlation function as described above. If three receiving stations report the time of arrival, three time difference values are calculated. Knowing the location of the receiving: stations RS1, RS2 and RS3 and the propagation speed of the signal transmitted from the wireless communications device 2, (i. e., the speed of light) the central processing site can determine the geolocation of the wireless communications device.
Generally, the time difference of arrival of the same signal at any two receiving stations is a constant (ignoring motion of the wireless communications device) and assuming flat terrain, yields a locus of points along a hyperbola. If the terrain is not flat, a hyperbola of rotation is the surface to be considered as it intersects the terrain. For example, with continued reference to FIG. 1, the possible locations of a wireless communications device transmitting a signal that arrived at receiving station RS1 at t1 and arrived at receiving station RS2 at t2 is defined by the locus of points comprising a hyperbola curve 6, where the curve is defined by t2−t1=k1, where k1 is a constant. With only two receiving stations using a TDOA method, it is generally not possible to determine a precise location for a target unit, but rather only a locus of points along a curve, such as the curve 6. Therefore, TDOA systems generally use at least three receiving stations to make a geolocation determination. For example, if the same signal is also received and time tagged by the third receiving station RS3, the central processing site computes two additional curves, based on the time difference of arrival of the signal at the three receiving sites taken in pairs. One such curve is determined by the time difference of arrival between receiving stations RS1 and RS3 (curve 7), and the other is determined by the time difference of arrival between receiving stations RS2 and RS3 (curve 8). The intersection of the curves 6, 7 and 8 is the geolocation of the wireless communications device 2.
It is known that in certain situations it may be possible to determine the location of the wireless communications device using the TDOA results from three receiving stations by calculating only two time difference of arrival values. The two generated hyperbolae will generally intersect at two points and thus the mobile location is not uniquely determinable. However, when the two points of intersection are overlaid with an area map showing various man-made and natural features, it may be possible to eliminate one of the two intersections as a possible mobile location. For example, if it is assumed that the wireless communications device is on a road, and only one of the two intersections occurs along a road, then the other potential location is eliminated from consideration. Alternatively, one of the receiving sites may be able to determine the sector of the azimuth plane (or more precisely the angle of arrival) from which the wireless communications device signal is received. When coupled with the two possible locations at the intersection of the two hyperbolae, this angle of arrival information allows for the selection of the correct location from the two possible locations.
As understood by those skilled in the art, the TDOA calculations may be adjusted for factors that affect the results, such as local terrain effects, time reference, etc.
Note that the prior art technique described above determines the TDOA between two signals received at spaced-apart receiving sites by performing time domain operations to first generate the cross-correlation function of the two received signals. The cross-correlation function represents the time offset between receipt of the two signals at their respective receiving sites. The peak value of the function identifies the minimum time offset, or the highest correlation of the two signals. Thus the time difference of arrival of the two signals at their respective base stations equals the time offset at the function peak.
To determine the cross-correlation function according to the prior art, the segment of both signals for which the TDOA is to be calculated must be transmitted to the central processing site, or at a minimum the segment of one received signal must be transmitted to the other receiving site for processing there. The cross-correlation function can be determined only by processing these received signal segments. Disadvantageously, there must be a relatively wideband transmission path between the various receiving sites, and the central receiving site, if applicable, to carry these signal segments and allow the calculation of the TDOA value in a relatively short period. Recall that in one embodiment the geolocation is being determined in response to an emergency (E911) call placed from the wireless communications device and thus time is of the essence in calculating the TDOA and then the caller's position.
Once the cross-correlation function is determined, it is necessary to find the function peak, which defines minimum time offset or the highest correlation of the two signals. This is typically carried out by sampling the cross-correlation function and comparing the sample values until the peak is found. In this search process, there can be some difficulty in accurately determining the peak's location because the peak is unlikely to fall directly on a time sample. Thus, if time difference of arrival values are to be determined more precisely than the sampling period, the cross-correlation function must be interpolated between sample periods to find the function peak. Alternatively, the cross-correlation function can be over sampled during the peak search process, but this is accomplished at the expense of increased computational overhead and time.